Detection of specific gaseous atoms or molecules in a gas sample may today be performed using any of a large number of different devices, ranging from complex technical systems like, for example, mass spectrometers and gas chromatographs, to small and relatively simple sensors like, for example, sensors measuring the thermal conductivity of a gas. Most of these devices are based on measurement of a physical or chemical property of gaseous atoms or molecules, whereas some of them instead are based on measurement of the actual presence of specific gaseous atoms or molecules.
For example, one device that may be used for detection of specific gaseous molecules in a gas sample and that is based on measurement of the actual presence of specific gaseous molecules, is described in SE 387444. More specifically, SE 387444 describes a gas sensitive sensor that may be used for detection of hydrogen gas.
There are many different applications in which devices for detection of hydrogen gas are necessary, useful or desired to utilize. For example, such a device may be utilized as a leak detector in systems using hydrogen gas, as a leak detector in systems and methods using hydrogen gas as a tracer gas for testing and/or locating leaks, or as an alarm detector to indicate the presence of hydrogen gas within, for example, industries using hydrogen gas or gas mixtures containing hydrogen gas (such as petrochemical industries, electrochemical industries, gasworks) for the purpose of preventing explosions.
The gas sensitive sensor described in SE 387444 comprises a catalytic metal layer constituting a metal electrode, a semiconductor layer and an insulator layer arranged between the catalytic metal layer and the semiconductor layer. Since this sensor comprises a semiconductor structure, it is herein denoted as a gas sensitive semiconductor sensor. The catalytic metal layer is made of any of the platinum metals palladium, nickel and platinum or an alloy containing at least 20% palladium by atomic weight.
The working principle of the semiconductor sensor in SE 387444 for detection of hydrogen gas is based on the fact that some of the platinum metals, especially palladium, are able to adsorb hydrogen gas molecules and dissociate adsorbed hydrogen gas molecules on their surfaces, to dissolve and allow penetration of hydrogen atoms thus formed and to adsorb hydrogen atoms at their surfaces. The term “catalytic metal” is herein used to denote a metal or an alloy being capable to dissociate hydrogen gas molecules and to absorb the hydrogen atoms thus formed.
The basic working principle of the semiconductor sensor in SE 387444 will now be described for the case when the sensor is utilized for detection of hydrogen gas. When the semiconductor sensor in SE 387444 is exposed to hydrogen gas molecules, the catalytic metal layer may adsorb some of them on its outer surface arranged to freely communicate with the ambient atmosphere. The adsorbed hydrogen gas molecules may then be dissociated on the outer surface and the hydrogen atoms thus formed may be absorbed into the catalytic metal layer. Some of the absorbed hydrogen atoms will subsequently be adsorbed at the interface between the catalytic metal layer and the insulator layer after diffusion through the catalytic metal layer.
Furthermore, it is well established that hydrogen atoms adsorbed at the interface between the catalytic metal layer and the insulator layer are polarized with the positive end facing the insulator layer (Lundström, I., Sensors and Actuators 1, 403 (1981)). The polarization implies that hydrogen dipoles are produced. The hydrogen dipoles generate an electrical field that shifts the effective work function of the catalytic metal layer. In consequence of the shift of the effective work function of the catalytic metal layer, the electrical function of the semiconductor sensor is influenced, i.e. a voltage shift in the characteristics of the semiconductor sensor is generated, and this influence is utilized for the detection of hydrogen gas. This sensing principle is herein referred to as the “hydrogen dipole transducer principle”.
The shift of the effective work function of the catalytic metal layer generated by hydrogen atoms adsorbed at the interface between the catalytic metal layer and the insulator layer may not only be utilized for detection of presence of hydrogen gas in a gas sample, but also for measurement of the concentration of hydrogen gas in a gas sample. The magnitude of the shift is determined by the number of hydrogen atoms adsorbed per unit area, i.e. the density of hydrogen atoms, at the interface between the catalytic metal layer and the insulator layer. Since the amount of hydrogen gas molecules in a gas sample and the amount of hydrogen atoms adsorbed at the interface between the catalytic metal layer and the insulator layer equilibrate after a certain time, the magnitude of the equilibrium shift may be utilized as a measure of the concentration of hydrogen gas molecules in a gas sample. However, the time before equilibrium is achieved between the amount of hydrogen gas molecules in a gas sample and the amount of hydrogen atoms adsorbed at the interface between the catalytic metal layer and the insulator layer is usually relatively long. For that reason, it is preferred to utilize the rate with which the effective work function is shifted, i.e. the rate with which the output signal is shifted, before an equilibrium shift is achieved, as a measure of the concentration of hydrogen gas molecules in a gas sample. Furthermore, the magnitude of the shifting rate of the effective work function and the magnitude of the equilibrium shift at a certain concentration of hydrogen gas molecules in a gas sample are of course dependent on the sensitivity to hydrogen gas molecules of the sensor.
A hydrogen gas sensitive semiconductor sensor working based on the same working principle as the sensor in SE 387444, i.e. based on the so-called hydrogen dipole transducer principle, is hereinafter denoted as a “hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole transducer principle”.
Hydrogen gas sensitive semiconductor sensors working based on the hydrogen dipole transducer principle are known to have a very high selectivity for hydrogen gas when operated up to around 150° C. However, it has been shown that such sensors have high sensitivities also to other gaseous hydrogen-containing molecules, such as alcohols and unsaturated hydrocarbons, when operated at higher temperatures. For example, sensitivity to methanol and ethanol of sensors working based on the hydrogen dipole transducer principle have been shown when operated at temperatures above 150° C. (Ackelid, U. et al, Metal-Oxide-Semiconductor structures with thermally activated sensitivity to ethanol vapour and unsaturated hydrocarbons, Proc. 2nd Int. Meet., Chemical Sensors, Bordeaux 1986, pp 395-398). In the same way as hydrogen gas molecules, such gaseous hydrogen-containing molecules are then adsorbed and dissociated on the outer surface of the catalytic metal layer and hydrogen atoms thus formed are absorbed into the catalytic metal layer.
Thus, semiconductor sensors working based on the hydrogen dipole transducer principle may not only be utilized for detection of hydrogen gas molecules, but also for detection of other gaseous hydrogen-containing molecules. However, when such sensors are to be utilized for detection of hydrogen gas, they are preferably operated at temperatures below 150° C. in order to obtain as high selectivity for hydrogen gas as possible and to avoid sensitivity to other gaseous hydrogen-containing molecules to which the sensor is sensitive when operated at higher temperatures.
The characteristic sensitivity to hydrogen gas molecules of a specific hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole transducer principle depends on the catalytic property of the catalytic metal layer, i.e. the ability of the catalytic metal layer to dissociate hydrogen gas molecules on the outer surface and to absorb hydrogen atoms thus formed. The reason for why the catalytic property of the catalytic metal layer influences the sensitivity is of course that it influences the number of hydrogen atoms that may be adsorbed per unit area, i.e. the density of hydrogen dipoles, at the interface between the catalytic metal layer and the insulator layer at a certain concentration of hydrogen gas in a gas sample.
However, the sensitivity of a hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole transducer principle may be reduced by, for example, oxidization of the outer surface of the catalytic metal layer. Oxygen in the surroundings of the semiconductor sensor may adsorb on, or bond to, the outer surface of the catalytic metal layer. Thereby, the number of adsorption sites, to which hydrogen gas molecules may adsorb, on the outer surface of the catalytic metal layer is being reduced concurrently with the number of molecules and atoms of oxygen being increased on the outer surface of the catalytic metal layer. When the number of adsorption sites, to which hydrogen gas molecules may adsorb, is reduced, the number of hydrogen gas molecules that may be adsorbed and dissociated on the outer surface of the catalytic metal layer and the number of hydrogen atoms that may be absorbed into the catalytic metal layer, at a certain concentration of hydrogen gas in a gas sample, are reduced. Thereby, the number of hydrogen atoms that may be adsorbed per unit area at the interface between the catalytic metal layer and the insulator layer, i.e. the number of hydrogen dipoles that may be achieved, at a certain concentration of hydrogen gas in a gas sample, is reduced. This implies that the sensitivity then is reduced.
Most gas samples subject to analysis regarding hydrogen gas contain air or oxygen gas. Thereby, the above mentioned reduction of the sensitivity of a hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole transducer principle is of frequent occurrence.
Furthermore, there are also other contaminants that may adsorb on, or bond to, the outer surface of the catalytic metal layer and thereby influence the sensitivity in the same way as oxygen. One example of such a contaminant is carbon monoxide, which also is present in many gas samples. In addition, hydrogen sulphide may bond to the outer surface of the catalytic metal layer.
When a hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole transducer principle is utilized under such detection conditions that oxygen and/or other contaminants may adsorb on, or bond to, the outer surface, the sensitivity of the sensor typically decays with sensor age due to oxidization or contamination by other contaminants of the outer surface. Usually, the life of a hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole transducer principle is substantially reduced under such detection conditions.
One way of counteracting the sensitivity reduction by oxygen and other contaminants is to purify the gas samples to be tested by the sensor from oxygen and other contaminating substances. Thereby, any influences on the sensitivity by oxygen and other contaminants are substantially counteracted and the life of the sensor is increased. However, such purification is relatively difficult and complicated to perform and results in that the total analysis time is lengthened since an extra step of sample preparation then is added to the analysis procedure.
Another way of counteracting the sensitivity reduction by oxygen and other contaminants is described in U.S. Pat. No. 6,484,563. According to the method described in U.S. Pat. No. 6,484,563, the semiconductor sensor is exposed to a gas sample during a detection interval. Each detection interval is preceded by a time interval during which the semiconductor sensor is kept in a surrounding preconditioning gas atmosphere containing negligible amounts of oxygen, hydrogen and carbon monoxide. The preconditioning time interval is much longer than the detection interval. The result during a preconditioning time interval is that essentially all oxygen and carbon monoxide, if any, adsorbed on the outer surface of the catalytic metal layer during a preceding detection interval are removed. Thereby, the above mentioned influences on the sensitivity by oxygen and other contaminants are substantially counteracted and the life of the sensor is increased. However, this method requires the use of equipment for modification of the atmosphere surrounding the semiconductor sensor.
The two above mentioned ways of counteracting the sensitivity reduction by oxygen and other contaminants may thus be utilized for increasing the life of a hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole transducer principle. However, these two ways do not increase the initial sensitivity of such a sensor. It is for most applications preferred, or required, that not only the life of a hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole transducer principle is as long as possible, but also that the initial sensitivity is as high as possible. The term “initial sensitivity” refers herein to the sensitivity of a sensor during an initial time period during an initial use of the sensor, i.e. the sensitivity of a “new” and not previously used sensor.
An increased initial sensitivity as well as an increased life of a hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole transducer principle may of course be achieved by modifying the catalytic property of the catalytic metal layer. One way to modify the catalytic property of the catalytic metal layer of a hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole principle in order to increase the initial sensitivity as well as the life is to utilize another catalytic metal assigning a higher initial sensitivity to the sensor. However, in most such sensors the catalytic metal known to assign the highest initial sensitivity to a hydrogen gas sensitive semiconductor sensor is already used today. Another way to modify the catalytic property of the catalytic metal layer of a hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole transducer principle in order to increase the initial sensitivity as well as the life is to increase the temperature of the outer surface during hydrogen gas detection. It is known that when the temperature is increased to above 150° C., the sensitivity to hydrogen gas is increased. However, the selectivity for hydrogen gas is reduced when the temperature is increased to above 150° C. and the sensitivity to other gaseous hydrogen-containing molecules, such as those mentioned above, is then increased.
There is still a need for a reliable way of increasing the initial sensitivity to hydrogen gas molecules as well as the life of a hydrogen gas sensitive semiconductor sensor working based on the hydrogen dipole transducer principle.